1Who defines celestial and terrestrial reference systems?
The International Astronomical Union (IAU) and International Union of Geodesy and Geophysics (IUGG)
2Who maintains reference frames?
The International Earth Rotation and Reference Systems Service (IERS)
3What is the difference between a reference system and a reference frame?
A system is a set of concepts and models; a frame is its realization with origin, axes, and scale
4What is the origin of the International Celestial Reference System (ICRS)?
The solar system barycenter (SSB)
5What defines the axes of the ICRS?
Positions of extragalactic radio sources and the equatorial plane
6What is measured in celestial reference frames?
Right ascension and declination
7What is the epoch of the standard celestial reference frame?
J2000.0 (January 1, 2000)
8Which catalogs formed the basis of celestial reference frames?
FK4 (1963), FK5 (1988), FK6 (2000, tied to ICRF)
9What is the ICRF based on?
Over 3,400 compact radio sources, with 295 defining sources
10What is the origin of the International Terrestrial Reference System (ITRS)?
The Earth’s center of mass, including oceans and atmosphere
11What is the orientation of ITRS based on?
IERS Reference Pole (IRP) and IERS Reference Meridian (IRM), tied to BIH at epoch 1984.0
12What ensures stability of ITRS over time?
It has no net rotation with respect to Earth’s crust
13What is the realization of ITRS?
The International Terrestrial Reference Frame (ITRF)
14What does the ITRF consist of?
A global set of space geodetic observing stations with precise coordinates and velocities
15What is the latest ITRF solution (as of now)?
ITRF2014
16What is GRS80?
The Geodetic Reference System of 1980, a geocentric system defined by IUGG Resolution No. 7
17What is WGS84 based on?
The GRS80 ellipsoid, with the same equatorial radius and similar constants
18How was WGS84 originally realized?
By Doppler observations from TRANSIT satellites and TRANET stations
19What is WGS84 used for today?
As the reference ellipsoid for GPS and GNSS positioning
20What are the main constants of WGS84?
Semi-major axis a = 6,378,137.0 m; Flattening f = 1/298.257223563; GM = 3.986005×10⁸ m³/s²; Angular velocity = 7.292115×10⁻¹¹ rad/s; J₂ = −484.16685×10⁻⁶
21What is the latest realization of WGS84?
WGS84 (G1762), the sixth update